Narrow linewidth BBO optical parametric oscillator utilizing extraordinary resonance

ABSTRACT

Optical parametric oscillators using non-linear crystals cut for the type I interaction have an extraordinary pump beam which generates ordinary signal and idler beams. The use of an optical element to rotate the signal beam before sending this signal beam to a grating element allows for the grating element to have its dispersion plane oriented coplanar with the extraordinary crystal plane of the non-linear crystal. In this manner, for non-linear crystals which have relatively narrow acceptance angles, such as BBO crystals, the reduction in angular aperture of the parametric gain in the extraordinary plane will produce a reduction in the linewidth of the produced output. The grating translates the angular variations into variations in wavelength. If no rotator was used in the optical parametric oscillator, then in order to get the maximum efficiency out of the grating, the dispersion plane of the grating should be orthogonal to the extraordinary plane of the crystal, and therefore the narrow angular acceptance of the crystal would not result in a narrow output linewidth.

BACKGROUND OF THE INVENTION

This invention relates to optical parametric oscillators (OPOs). Opticalparametric oscillators use non-linear crystals in order to tune anoutput over a range of frequencies. The non-linear crystals are placedwithin a resonator and driven by an intense pump radiation to generatetunable monochromatic light. OPOs use a three-wave process in which thehigh-frequency pump ω_(p) is decomposed into a signal wave ω_(s) and anidler wave ω_(i). The relationship of the pump, signal, and idler isgoverned by the conservation of energy and momentum. Energy conservationrequires that the sum of the generated energies, and thereforefrequencies, equal that of the pump. ω_(p) =ω_(s) +ω_(i). Momentumconservation is shown by the phase-making relationship k_(p) =k_(x)+k_(i). The values k_(p), k_(s), and k_(i) are the momentum vectors forthe pump, signal, and idler, respectively, and are related by thewavelength λ of each wave by the relation k=2πn/λ, where n is the indexof refraction.

In uniaxial birefringent crystals such as β-BaB₂ O₄ (BBO), the index caneither be ordinary or extraordinary (n_(o) or n_(e)). If ordinary, thepolarization vector of the light beam within the crystal is orthogonalto the optic axis of the crystal. In this plane, there is no angularrelationship to the index. If extraordinary, the polarization vector ofthe light beam is in the plane of the optic axis, and therefore there isan angular relationship for the index. Momentum matching is achieved byrotating the crystal in the extraordinary plane, thereby varying theindex and its associated k vector, of one of the light waves.

In many applications, it is desired that the output of the opticalparametric oscillator have a narrow linewidth. Applications inspectroscopy and photochemistry may require linewidths of less than 0.1cm⁻¹. Additionally, it is important that the optical parametricoscillator be efficient. This is especially true since some non-linearcrystals such as BBO are typically pumped close to their damagethreshold.

SUMMARY OF THE INVENTION

Gratings can be used to tune a beam such as the signal or idler which isfed back in the optical parametric oscillator. Angular variations ofthis beam translate into wavelength variations in the feedback signalbecause of the grating dispersion.

Some non-linear crystals, such as BBO cut for type I interaction, have anarrow acceptance angle in the extraordinary axis. In type Iinteractions, the pump beam is an extraordinary wave, and the signal andidler beams are ordinary waves. The non-linear crystal is oriented sothat there is an angle θ between the optic axis of the non-linearcrystal and direction of propagation of the extraordinary beam so thatthe parametric amplification of the desired signal occurs. This angle θis determined by the phase matching requirements of the crystal. Theacceptance angle is the angular range about this angle θ for which adiverging laser beam will interact.

The present invention involves placing the dispersion plane of thegrating in the extraordinary plane defined by the non-linear crystal soas to use the narrow acceptance angle of the non-linear crystal to helpnarrow the linewidth of the output.

For type I interactions, an extraordinary pump beam produces ordinarysignal and idler beams. In this configuration, it would not be obviousto arrange the grating such that its dispersion plane is in theextraordinary plane of the crystal. The grating has its greatestefficiency if the grating lines are perpendicular to the ordinarypolarized signal beam. This causes the dispersion plane of the gratingto be orthogonal to the extraordinary plane of the crystal.

For a type I interaction in the non-linear crystal, an embodiment of thepresent invention involves rotating the polarization of the signal beamfrom the ordinary to the extraordinary plane so that the grating has ahigh efficiency when it is aligned with its dispersion plane in theextraordinary plane defined by the non-linear crystal. In this manner,the grating is arranged with its grating lines orthogonal to theextraordinary plane so that the polarization of the ordinary productbeam is perpendicular to the grating lines of the grating. The narrowacceptance angle of the extraordinary pump beam in the extraordinaryplane of the non-linear crystal prevents pump waves having an angularcomponent in the extraordinary plane outside of the acceptance anglefrom decomposing into signal and idler photons. For this reason, theangular variations in the extraordinary plane about the cavity axis ofthe OPO of the product beam is reduced. The gratings convert thisreduced angular variation into a reduced linewidth of the product beam.In effect, the rotation of the product beam and orientation of thegrating allows the angular filter effect of crystal to translate into anarrower linewidth.

Benefits can be obtained to a lesser extent by rotating the product beamaway from the ordinary polarization, but not completely into theextraordinary plane when the grating lines are orthogonal to theextraordinary plane of the crystal. This partial rotation allows theproduct beam to be more perpendicular with respect to the grating linesthan parallel. The optical element used to rotate the polarization ofthe ordinary beam may cause a rotation which depends upon the wavelengthof the ordinary signal beam. Since the ordinary signal beam can be tunedby the optical parametric oscillator, the polarization of the ordinarysignal beam may not be completely into the extraordinary plane definedby the non-linear crystal.

The present system has benefits over the use of beam-expanding prisms inthe optical parametric oscillator. In lasers, prism beam expanders havebeen used to obtain a narrow linewidth. The prism beam expander causeslosses, however. Lasers can overcome this loss by being pumped harder.In optical parametric oscillators, some crystals, such as BBO crystals,are pumped close to their damage threshold. This means that any lossescaused by the prism beam expanders will have to be offset by an increasein pump energy, thereby increasing the propensity for crystal damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading of the following detailed descriptionin conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of an optical parametric oscillatorwithout a rotator;

FIG. 2 is a diagrammatic view of an optical parametric oscillator with arotator and diffraction grating;

FIG. 3 is a diagrammatic view of an optical parametric oscillator with arotator and Littrow-mounted grating;

FIG. 4 is a diagrammatic view of an optical parametric oscillator with arotator and Littrow prism;

FIG. 5 is a diagram illustrating the acceptance angle;

FIG. 6 is a graph showing the efficiency of the interaction versus theangle from the optic axis of the non-linear crystal to illustrate theacceptance angle; and

FIG. 7 is a graph showing the grating efficiency versus wavelength fordifferent polarizations.

Similar elements among the different figures are labeled the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagrammatic view of an optical parametric oscillator 10without a rotator. This optical parametric oscillator 10 has anon-linear crystal 12. For a type I interaction (eoo) in the opticalparametric oscillator, the pump beam for the pump source 14 is polarizedin the extraordinary plane of the non-linear crystal 12. Theextraordinary plane of the non-linear crystal 12 is a plane of rotationof the optic axis c of the non-linear crystal 12, when the non-linearcrystal 12 is rotated to be tuned at different angles. The signal andidler beams produced in the non-linear crystal 12 both have ordinarypolarization so their polarization is orthogonal to the extraordinaryplane of the crystal 12.

As shown in FIG. 1, the extraordinary plane of the crystal 12 is shownas a uniaxial crystal in which the index can be either ordinary orextraordinary. For the ordinary polarized beams, the signal and theidler, there is no angular relation to the index because thepolarization of the light beam inside the crystal is orthogonal to theoptic axis c of the crystal 12.

Rotating the crystal 12 in the extraordinary plane changes the angle θbetween the direction of the propagation of the beams in the opticalcavity and the optic axis c. Since in this situation the pump beam ispolarized in the extraordinary plane, varying the angle θ varies theindex n_(e) and therefore varies the phase matching relationship, k_(p)=k_(x) +k_(i).

The optical parametric oscillator 10 also uses a grating 16 and tuningmirror 18 to feed back a beam into the crystal 12. In the preferredembodiment, the signal beam is fed back into the crystal 12. By rotatingthe tuning mirror 18, the first order reflections off the grating 16 canbe sent back along the axis of the optical parametric oscillator 10. Thetuning mirror 18 and the rear mirror 20 define the cavity of the opticalparametric oscillator 10. A signal beam will reflect between these twomirrors to seed the parametric interaction in the non-linear crystal 12.The non-linear crystal 12 and the tuning mirror 18 are both tuned in amanner that the desired signal beam resonates in the OPO 10. Zero orderreflections off the grating 16 provide the output of the OPO 10.

The dispersion plane of the grating 16 is orthogonal to theextraordinary plane of the non-linear crystal 12. This orientation isused because the signal beam is polarized orthogonal to theextraordinary plane of the crystal. As shown in FIG. 7, for the relevantsignal wavelengths, the range of around 450-750 nm, the grating is muchmore efficient if the input beam has its polarization perpendicular tothe grating lines 16a of the grating 16. If the grating 16 is arrangedwith its grating lines perpendicular to the ordinary polarization of thesignal beam, the dispersion plane is orthogonal to the extraordinaryplane of the non-linear crystal 12.

The tuning mirror 18 is actually placed substantially on top of thegrating 16. As shown in FIG. 1, the pump from the pump source 14 is sentthrough the non-linear crystal in two passes. The mirrors 22 and 24 arereflective for the pump beam wavelengths, and transmissive for thesignal wavelengths. The pump beam reflects off the mirror 22 through thenon-linear crystal 12 to the mirror 24 and then returns through thenon-linear crystal and mirror 22 out back to the pump source 14. Thesignal beam passes through both mirrors 22 and 24 so that the signalbeam resonates in the cavity formed between mirror 20 and tuning mirror18.

The linewidth of the signal beam can be determined with a model similarto the model given by Brosnan and Byer in their paper "OpticalParametric Oscillator Threshold and Linewidth Studies," IEEE Journal ofQuantum Electronics; Vol.QE-15, No. 6; June 1979; pp. 415-431, which isincorporated herein by reference. This model assumes that for thegrating oscillator two mechanisms affect the linewidth: signal beamdiffraction; and pump beam aperture. The diffraction term is dominatedby the resonated spot size of the signal, while the aperture term isrelated to the pump beam spot size.

The intrinsic linewidth of the OPO crystal Δv_(c) is only important whenit is the same order of magnitude as the grating linewidth. The crystallinewidth is usually a few orders of magnitude broader than the gratinglinewidth, so that the relation ##EQU1## simply reduces to

    Δv.sub.opo =Δv.sub.G

The grating linewidth is the sum of the diffraction and aperture terms

    Δv.sub.G =Δv.sub.D +Δv.sub.A

The diffraction limited linewidth is derived from considering the spotsize of signal and the propagation of the beam inside a resonator oflength L. There is a resulting diffraction limited angle which, inconjunction with the grating resolution, determines this linewidth term.The diffraction linewidth is given as ##EQU2## where the terms aredefined: α,β angles of grating incidence and diffraction

r=grating reflections per round trip

M=linear magnification

L=cavity length

λ_(S) =signal wavelength

w_(s) =resonated spot size.

The resonated signal spot size evolves during the buildup time of theOPO. Its final steady state value is that of the pump. However, duringthe formation of the signal, it is significantly smaller than the pumpsize w_(p), and is given by a cubic equation ##EQU3## The diffractionlimited linewidth is determined by this spot size, which sets thediffraction angle of the system during the formation of the pulse. For apump waist of 1.0 mm and a cavity length of 15 cm, the resonated spotsize at 600 nm is 0.31 mm.

The diffraction angle assumes a steady state value early in theformation of a signal beam. The diffraction limited angle for a guassianbeam is ##EQU4## Using multiple gratings or a grazing incidence designreduces the diffraction angle by the number of grating reflections percavity transit. The factor r accounts for this. r is 1 for a Littrowgrating, 2 for grazing incidence, or 3 for dual grating. The beamexpander reduces the diffraction angle by the linear magnification. Fora brewster angle prism, the linear magnification is given as M=n, wheren is the angle of diffraction. For N prisms, M=M^(N). The resolution ofthe grating in grazing incidence is given as: ##EQU5## The diffractionlinewidth expression is then ##EQU6## The factor (21n2)^(1/2) in thelinewidth expression sets the 1/e² value to a full width at half-maximum(FWHM) linewidth. For a pump waist of 1 mm, a corresponding resonatedsignal spot size of 0.31 mm and no beam expansion, the diffraction angleat 600 nm is 330 microradians. This yields a diffraction linewidth of0.12 cm⁻¹ when a 2400 line/mm grating at 88.4° is utilized in a cavity15 cm long.

The aperture linewidth comes from considering the pump waist and a raygeometry inside the cavity. Off-axis wavelengths are amplified in thegain medium and tend to broaden the linewidth. The aperture linewidthexpression is ##EQU7## wherein the terms are defined as α=grating angleof incidence

w_(p) =pump beam spot size

a=groove spacing

p=number of passes

D=effective cavity length.

The angle over which the pump beam will amplify off-axis gratingreflections is determined from the cavity geometry. The length D of thecavity is an effective length. This effective length is the length of anopen cavity which would have the same ray path as a cavity with higherindex materials such as beamsplitters, a crystal and a beam expander.

The grating on each pass reflects the off-axis radiation out of theamplifying region. The number of passes is assumed to be the number ofcavity transits during the pump duration τ multiplied by the number ofgrating reflections ##EQU8##

Again, we use the coefficient r to account for the case of tworeflections per pass in a grazing incidence design, and threereflections per pass in a dual grating system. The summation of all thepasses through the aperture w_(p) is (P+1)P/2. The net effect of themultiple passes on the grating is a reduction of the aperture angle. Aprism beam expander will also reduce the aperture angle by themagnification M. The aperture angle is given as ##EQU9##

For a grazing incidence oscillator with the effective length of 14 cm,ten grating passes, no beam expansion, and a 1 mm pump waist, theaperture angle is 130 microradians.

The grating operating in the first order has a dispersion relation##EQU10## Therefore, the aperture linewidth is ##EQU11## For the examplesystem mentioned above, the aperture linewidth is 0.05 cm⁻¹ at 600 nm.This gives a fairly typical total linewidth value of 0.17 cm⁻¹ for agrazing incidence grating optical parametric oscillator using ordinarysignal resonance. In ordinary signal resonance, the beam has apolarization orthogonal to the extraordinary plane, and the dispersionplane of the grating is orthogonal to the extraordinary plane of thenon-linear crystal 12.

A summary of the laboratory results is given in Table 1. Examples 1-3give the laboratory results for the ordinary resonance case. Looking atExample 1, we see that the typical measured linewidth at 543 nm is onthe order of 0.25 cm⁻¹ for a grazing incidence single grating systemwith no expansion elements. The model predicts 0.24 cm⁻¹ linewidth. Byexpanding the intracavity beam with prisms, we can reduce the measuredlinewidth, as shown in Example 2, to 0.07 cm⁻¹. Here, the grating anglehas to be reduced to 84.75° to overcome the losses of the prism beamexpander.

We also used a second Littrow grating in conjunction with the beamexpander to narrow the linewidth. In this case, Example 3, we had toreduce the angle of grating incidence to 82.6° to increase thediffraction efficiency, because the Littrow grating adds a loss. Theresult is nearly the same as the single grating/expander case. Themeasured linewidth is 0.07 cm⁻¹, but the threshold is about 70 mJ asopposed to 40 mJ for the grating-only case of Example 1.

The reduction of the aperture and diffraction angles and the increasingof the grating dispersion are keys to achieving the narrow linewidth,but the losses imposed by the expanders and the dual gratings requiresan increased pump energy beyond the safe operating limits of thenon-linear crystal.

Achieving less than 0.1 cm⁻¹ linewidth in a low-loss resonator with onlya single diffraction grating and no beam expander is possible byresonating the signal radiation in the extraordinary crystal plane. FIG.2 is a diagrammatic view of an optical parametric oscillator 26 with arotator 28 and diffraction grating 30. The non-linear crystal 32 is cutfor the type I interaction where an extraordinary pump is converted toan ordinary signal and idler beams. This means that the polarization ofthe pump coming into the non-linear crystal 32 is in the extraordinaryplane, and the polarization of the signal in the idler beams producedhave an ordinary polarization. The ordinary polarization has apolarization vector which is orthogonal to the extraordinary planedefined in the non-linear crystal 32.

The rotator 28 rotates the polarization of the signal and idler beamssuch that preferably the polarization of the signal beam is rotated intothe extraordinary plane. The diffraction grating 30 is arranged suchthat its dispersion plane is in the extraordinary plane defined by thecrystal 32. The grating 30 has grating lines perpendicular to theextraordinary plane of the non-linear crystal. This means that therotated polarization of the signal and idler beams are perpendicular tothe grating lines of the grating 30. For this reason, the grating 30will have a high efficiency.

FIG. 7 shows the grating efficiency versus wavelength for differentpolarization orientations of a 2400 lines/mm holographic grating. Asshown, the perpendicular polarization has a high efficiency in the rangeof about 450 nm to 750 nm. By rotating the polarization of the signalbeam, the high efficiency perpendicular orientation with respect to thegrating lines can be used at the same time that the dispersion plane ofthe grating is in the extraordinary plane defined by the crystal 32.

Alternate embodiments for the optical element 28 which is used to rotatethe polarization of the signal beams could be a zero order half-waveplate, a 90° polarization rotator, or a quartz beam rotator.

As described below, a non-linear crystal 32 with a narrow acceptanceangle, such as a BBO crystal cut for type I interactions, reduces theangular aperture for parametric gain in the extraordinary plane. Thisreduction of angular aperture acts an angular filter which reduces theangular variation about the cavity axis in the extraordinary plane atthe grating 30 and thus reduces the wavelength variation of the signalbeam as it is fed back in the optical parametric oscillator.

FIG. 5 is a diagram which illustrates the acceptance angle concept. Thisdiagram is shown in the extraordinary plane of the crystal. The cavityaxis 100 of the OPO forms an angle θ with the optical axis c of thenon-linear crystal. Both the cavity axis 100 and optic axis c are in theextraordinary plane of the non-linear crystal. Assuming that thenon-linear crystal is tuned so that the angle θ is a phase-matchingangle for the desired signal beam, an extraordinary pump beam will havea finite acceptance angle Δθ.

FIG. 6 is a graph showing an illustration of the acceptance angleconcept. This curve shows the phase synchronism relation ##EQU12## whichis the measure of the phase mismatch Δk for a crystal of length L. Theangular bandwidth (acceptance angle) is that angle defined by the fullwidth at half maximum (FWHM) of the phase synchronism expression.

The phase matching criteria is given as k_(p) =k_(s) +k_(i). The phasemismatch is a perturbation of the exact phase-matching and can bedenoted as k_(p) =k_(s) +k_(i) +Δk. The FWHM of the phase synchronismexpressed for a crystal of length L can be converted into an angle byrealizing that Δk can be expressed in terms of index change for the pumpand consequently an angular change for the pump beam.

The phase synchronism curve depicts the actual interaction strength ofthe parametric process or the parametric gain as a function of angle.The crystal in the extraordinary plane will aperture the pump beam bylimiting its efficiency in the process to the angle. In the ordinaryplane, this synchronism expression is not valid and the pump can provideparametric gain to signals within the entire divergence angle of thepump which can be much larger than the acceptance angle.

In FIG. 2, the grating dispersion plane is operating in the criticalphase-matching, or extraordinary, plane of the crystal. As discussedabove, the crystal acceptance angle must be taken into account whenconfiguring the system in this orientation. BBO, it should be noted, hasa smaller acceptance angle than most other types of crystals. We cancalculate the single-pass acceptance angle for a type I crystal oflength L: ##EQU13## where n_(op) =ordinary index of pump

n_(ep) =extraordinary index of pump

θ=phase-matching angle of crystal

λ_(p) =pump wavelength

n_(ep) (θ)=the extraordinary index as a function of θ.

For a 15 mm BBO crystal for type I interaction pumped at 355 nm, thissingle-pass acceptance angle is 175 microradians. If we double-pass thepump, as shown in FIG. 2, the angle is reduced to 88 microradians,significantly smaller than the diffraction and aperture terms calculatedabove.

The crystal serves the function of an angular, as well as spectral,filter. First, only wavelengths within the intrinsic crystal spectralbandwidth will be amplified. Second, the signal waves propagatingoutside of the acceptance angle will not be amplified, even though theyfall within the spectral bandwidth. This is not the case in the ordinaryplane, where the pump k vector will match diverging signal k vectors.

In the non-critical or ordinary plane, the pump k vector does not changewith angle. Therefore, the off-axis components of the signal radiationwithin the spectral bandwidth of the device can be amplified if there isany corresponding pump component in the same cone angle. Since the pumpbeam is slightly divergent, there are off-axis pump components availableto amplify the signal. In the extraordinary plane, the pump beam kvector is dependent on the angle; thus off-axis colors will not beamplified outside of a finite acceptance angle shown in FIG. 6.

Looking again at FIG. 2, the grating 30 works in conjunction with thecrystal 32. The diffraction of light from the grating 30 caused by thenarrow ruling lines on the grating surface disperses the light in onlyone plane. This is the plane in which the light is sent back along theresonator optical axis 34. Since the amplified light cone angle isreduced significantly in the extraordinary plane, and the gratingdispersion is aligned in this plane, the resultant linewidth is narrowerwith the extraordinary resonance.

Returning to the equations for Δν_(d) and Δν_(A), we see that the modelcalculated the diffraction linewidth based upon the steady statediffraction angle and aperture linewidth based upon the reflection fromthe grating of off-axis rays into the gain volume. What we have done tothe model, in the case of the extraordinary resonance, is to convolutethe two angular bandwidths for the aperture and diffraction with thecrystal acceptance angle bandwidth: ##EQU14##

These modifications to the model yield a narrow acceptance angle whenthe crystal term is sufficiently smaller than the cavity term, but allowthe cavity terms to dominate in the cases of beam expanders and dualgratings. If we now look at the examples of Table 1, we see that themeasured reduction in linewidth due to the extraordinary resonancecorrelates favorably with the model. In Example 4, we used a 5°non-collinear pump geometry. The pump beam waist was 1.5 mm, and thegrating angle was 87.8°. At 600 nm, the measured linewidth was 0.07cm⁻¹, and the calculated linewidth was 0.08 cm⁻¹. If we had usedordinary resonance, the linewidth would measure 0.22 cm⁻¹. Examples 5-8in Table 1 are laboratory data from 480-700 nm, using a 1 mm pump waistand grating angle of 88.5°. As can be seen, a significant linewidthreduction occurs for the extraordinary resonance, and the measuredvalues correlate favorably to the calculated ones.

The present invention can also be used for type II interaction (eoeinteraction) wherein the feedback beam is the ordinary polarized beam.The BBO cut for type II interaction does not have quite as narrow anacceptance angle, so the linewidth narrowing benefits would not be asgreat.

Looking again at FIG. 2, the OPO 26 has a pump source 36 that preferablyprovides a 355 nm pump radiation directed into the non-linear crystal 32by the 45° turning mirror 38, which is a high reflector for the 355 nmpump radiation and a high transmitter for the 450-750 nm signalradiation. The non-linear crystal 32 is rotated by an angle θ from theoptic or c axis of the crystal 32 to the cavity axis 34 of the OPO 26.The plane of rotation of the crystal 32 is referred to as theextraordinary crystal plane of the non-linear crystal 32. Rotating thenon-linear crystal 32 in the extraordinary plane does not affect theindex seen by ordinary waves, which have their polarization orthogonalto the extraordinary plane.

Mirror 40 is a zero degree dichroic that retro-reflects the pumpradiation and transmits the signal radiation. The pump beam createsparametric fluorescence in the crystal 32 and successively amplifies theresonated signal in this crystal during the pump pulse duration. Theresultant signal is resonated between the rear cavity mirror 42 and thetuning mirror 44, both of which are broad band high reflectors from450-750 nm.

The signal is filtered spectrally twice on each pass by the first orderreflection from the grating 30. This grating 30 serves to output couplethe signal radiation from the zero order reflection along 46. Apreferred grating ruling density for narrow linewidth is 2400 lines/mmand the preferred angle of incidence is 88.5°. In the preferredembodiment, the optical element 28 is a zero order wave plate insertedbetween the grating 30 and the pump retro-reflector 40. This waveplate28 rotates by 90° the signal polarization vector from the ordinarycrystal plane to the extraordinary plane of the crystal. The signal beamreturning from the grating is rotated back into the ordinary plane bythe waveplate 28.

In the preferred embodiment, the non-linear crystal is a uniaxial BBOcrystal cut for type I interaction. The BBO crystal is available fromCrystal Technology, Inc. located in Palo Alto, Calif. The dichroicmirrors, retro-reflectors, rear mirror, and zero order waveplate areavailable from CVI, located in Albuquerque, N. Mex. The grating isavailable from Instruments SA, Edison, N.J. The tuning mirror isavailable from Newport Corp. located in Fountain Valley, Calif. The pumplaser is available from Continuum of Santa Clara, Calif., or SpectraPhysics Lasers of Mountain View, Calif.

FIG. 3 is a diagrammatic view of an OPO with a rotator 28' andLittrow-mounted grating 50. This embodiment is similar to the embodimentshown in FIG. 2; however, a Littrow-mounted grating 50 returns adiffraction order back to the OPO 52 and a partially transmissive mirror54 produces the output signal. In this case as well, the rotator 28'rotates the orientation of the signal beams from the polarizationorthogonal to the extraordinary axis to a polarization in theextraordinary axis, so that the dispersion plane of the grating 50 is inthe extraordinary plane of the non-linear crystal 32'. The grating linesof the Littrow-mounted grating 50 are orthogonal to the polarizations ofthe signal beam, so that the efficiency of the Littrow-mounted grating50 is optimized. The linewidth of the OPO 52 with the Littrow-mountedgrating is not as narrow as the linewidth of OPO 26 described in FIG. 2.However, looking again at FIG. 3, in this configuration, the narrowacceptance angle in the extraordinary crystal plane of the crystal 32'can be used to narrow the linewidth from the other orientation of thegrating. The Littrow grating is available from Instruments SA, ofEdison, N.J. The transmitting mirror is available from CVI ofAlbuquerque, N.Mex.

FIG. 4 is a diagrammatic view of an optical parametric oscillator with arotator 28" and a Littrow prism 60. The Littrow prism 60 is anotherembodiment for returning a diffraction order of the signal beam into theOPO 58. The Littrow prism is available from CVI of Albuquerque, N.Mex.

Looking again at FIG. 2, the rotator 28 does not have to rotate thesignal beam exactly 90° over the entire wavelength range of the signalwavelength. A zero order waveplate 28 can be set to rotate by 90° thepolarization of a center wavelength (i.e. 600 nm) of a 450-750 nm signaloutput range. Signals with a wavelength at either end of the outputrange would not have exactly a 90° rotation, so the signal beam sent tothe grating 30 would not necessarily be polarized in the extraordinaryplane of the non-linear crystal 32. The polarization would have arelatively large component in this plane, however, so that the grating30 could be somewhat efficient.

Additionally, it would be theoretically possible that the dispersionplane of the grating 30 be oriented at an orientation that is notorthogonal to the extraordinary plane of the crystal 32, yet not in theextraordinary plane. If the signal beam's polarization were rotated forthe grating's optimum efficiency, the grating's orientation would allowa partial linewidth narrowing effect as a result of the narrowacceptance angle of the non-linear crystal 32 in the extraordinaryplane. This partial effect is less than if the dispersion plane of thegrating was coplanar with the extraordinary plane of the crystal 32.

Various details of the implementation and method are merely illustrativeof the invention. It will be understood that various changes of detailsmay be within the scope of the invention, which is to be limited only bythe appended claims.

                                      TABLE 1                                     __________________________________________________________________________           Example 1                                                                           Example 2                                                                           Example 3                                                                           Example 4                                                                           Example 5                                                                           Example 6                                                                           Example 7                                                                           Example                      __________________________________________________________________________                                                     8                            Type   o wave                                                                              o wave                                                                              o wave                                                                              e wave                                                                              e wave                                                                              e wave                                                                              e wave                                                                              e wave                       Grating                                                                              Single 2400                                                                         Single 2400                                                                         Dual 2400                                                                           Single 2400                                                                         Single 2400                                                                         Single 2400                                                                         Single 2400                                                                         Single 2400                  Beam   no    6 prism                                                                             6 prism                                                                             no    no    no    no    no                           Expansion?                                                                    D (cm) 14    20    20    14    14    14    14    14                           L (cm) 15    30    30    15    15    15    15    15                           α                                                                              88°                                                                          84.75°                                                                       82.6°                                                                        87.80°                                                                       88.4°                                                                        88.4°                                                                        88.4°                                                                        88.4°                 λ (nm)                                                                        543   594   594   601.00                                                                              480   500   600   700                          w.sub.p (cm)                                                                         0.125 0.15  0.15  0.15  0.1   0.1   0.1   0.1                          w.sub.s (cm)                                                                         0.034 0.048 0.048 0.038 0.031 0.031 0.031 0.031                        measured                                                                             0.25  0.07  0.07  0.07  0.08  0.08  0.08  0.04                         linewidth                                                                     (cm.sup.-1)                                                                   model o                                                                              0.24  0.08  0.06  0.22  0.28  0.21  0.17  0.15                         wave (cm.sup.-1)                                                              model e                                                                              0.09  0.07  0.06  0.08  0.09  0.09  0.06  0.04                         wave (cm.sup.-1)                                                              __________________________________________________________________________

What is claimed is:
 1. An optical parametric oscillator comprising:anoptical cavity defined between two reflective elements; a non-linearoptical crystal having an extraordinary plane, said non-linear crystallocated in the optical cavity and adapted to generate a product beam ofordinary polarization in response to a pump beam of extraordinarypolarization, said product beam having a different wavelength from thepump beam; a grating in said optical cavity, said grating aligned sothat the dispersion plane of the grating is not orthogonal to theextraordinary plane of the non-linear crystal; and an optical element inthe optical cavity adapted to rotate the polarization of the productbeam from the ordinary polarization.
 2. The optical parametricoscillator of claim 1, wherein the grating is aligned so that thedispersion plane of the grating is substantially coplanar with theextraordinary plane of the non-linear crystal.
 3. The optical parametricoscillator of claim 2, wherein the optical element in the optical cavityis adapted to rotate the polarization vector of the product beam fromthe ordinary polarization to substantially the extraordinary crystalpolarization plane.
 4. The optical parametric oscillator of claim 1,wherein the optical element is a zero order half waveplate.
 5. Theoptical parametric oscillator of claim 4, wherein the zero order halfwaveplate is a 90 degree polarization rotator over the signal tuningrange.
 6. The optical parametric oscillator of claim 1, wherein theoptical element is a beam rotator.
 7. The optical parametric oscillatorof claim 1, wherein said product beam is the signal beam of thenon-linear crystal.
 8. The optical parametric oscillator of claim 1,wherein said product beam is the idler beam of the non-linear crystal.9. The optical parametric oscillator of claim 1, wherein said grating isa diffraction grating and wherein said reflective elements comprise arear mirror and a tuning mirror in the dispersion plane of thediffraction grating, said diffraction grating dispersing the productbeams into different orders, one of the orders being an output and thetuning mirror arranged to reflect another of the orders back toward thenon-linear crystal.
 10. The optical parametric oscillator of claim 1,wherein the non-linear crystal is cut for the TYPE I interaction. 11.The optical parametric oscillator of claim 1, wherein the non-linearcrystal has a narrow acceptance angle in the extraordinary plane. 12.The optical parametric oscillator of claim 11, wherein the non-linearcrystal is a BBO crystal.
 13. The optical parametric oscillator of claim1, further comprising a mirror in said cavity that is a high transmitterof the pump beam and a high reflector of the product beam.
 14. Theoptical parametric oscillator of claim 1, wherein the optical element isplaced between the non-linear crystal and the grating.
 15. The opticalparametric oscillator of claim 1, wherein the grating has grating linessubstantially orthogonal to the extraordinary plane.
 16. An opticalparametric oscillator comprising:a Littrow-mounted grating serving as amirror and a partially transmitting mirror forming an optical cavitytherebetween; a non-linear optical crystal having an extraordinaryplane, said non-linear crystal located in the optical cavity and adaptedto generate a product beam of ordinary polarization in response to apump beam of extraordinary polarization, said product beam having adifferent wavelength from the pump beam; and an optical element in theoptical cavity adapted to rotate the polarization of the product beamfrom the ordinary polarization, wherein the Littrow-mounted grating isaligned so that the dispersion plane of the grating is not orthogonal tothe extraordinary plane of the non-linear crystal.
 17. The opticalparametric oscillator of claim 16, wherein the Littrow-mounted gratingis aligned so that the dispersion plane of the grating is substantiallyco-planar with the extraordinary plane of the non-linear crystal. 18.The optical parametric oscillator of claim 17, wherein the opticalelement in the optical cavity is adapted to rotate the polarizationvector of the product beam from the ordinary polarization tosubstantially the extraordinary crystal polarization plane.
 19. Theoptical parametric oscillator of claim 16, wherein said Littrow-mountedgrating is a Littrow prism.
 20. The optical parametric oscillator ofclaim 16, wherein said Littrow-mounted grating disperses the productbeams into different orders, and wherein the Littrow-mounted grating isarranged to feed back one of the orders to the non-linear crystal. 21.The optical parametric oscillator of claim 16, wherein the non-linearcrystal is cut for the TYPE I interaction.
 22. The optical parametricoscillator of claim 16, wherein the non-linear crystal has a narrowacceptance angle in the extraordinary plane.
 23. The optical parametricoscillator of claim 22, wherein the non-linear crystal is a BBO crystal.24. A method of forming a narrow linewidth beam with a non-linearcrystal having an extraordinary plane comprising the steps of:amplifyingin said non-linear crystal a beam such that an amplified light cone issubstantially narrower in the extraordinary plane of the non-linearcrystal, said beam having an ordinary polarization; rotating thepolarization of the beam from the ordinary polarization; and dispersingsaid beam with a grating along a dispersion plane which is notorthogonal to the extraordinary plane of the non-linear crystal.
 25. Themethod of claim 24, wherein said non-linear crystal has a narrowacceptance angle in the extraordinary plane and wherein said amplifyingstep comprises pumping the non-linear crystal with a pump beam ofextraordinary polarization.
 26. The method of claim 25, wherein thenon-linear crystal is pumped with two passes of the pump beam.
 27. Themethod of claim 24, further comprising feeding back a portion of thebeam into the non-linear crystal.
 28. The method of claim 27, furthercomprising rotating the polarization of the feedback beam into theordinary polarization before the feedback beam re-enters the non-linearcrystal.
 29. The method of claim 24, wherein said dispersing stepcomprises dispersing along a dispersion plane substantially co-planarwith the extraordinary plane of the non-linear crystal.
 30. The methodof claim 24, wherein said rotating step comprises rotating thepolarization vector of the beam from the ordinary polarization tosubstantially the extraordinary crystal polarization plane.
 31. A methodof forming a narrow linewidth beam with a non-linear crystal having anextraordinary plane comprising the steps of:generating a beam ofordinary polarization in a non-linear optical crystal from a pump beamof extraordinary polarization, said beam having a different wavelengthfrom the pump beam; dispersing said beam with a grating along adispersion plane which is substantially co-planar to the extraordinaryplane of the non-linear crystal; and returning a portion of the beam tothe non-linear crystal.
 32. The method of claim 31, further comprisingrotating the polarization of the beam so that the beam has apolarization perpendicular to grooves of the grating.